1. Field of Invention
The present invention relates to the production technology of inorganic pigments lithopone, and more particularly to a method for producing nanometer lithopone from electrolytic zinc acid leaching residue.
2. Description of Related Arts
Inorganic pigments lithopone lithopone is also known as Xinbeibai, which is the mixed crystal of zinc sulfide and barium sulfate. Lithopone has much unique and excellent performance, and lithopone has formed a series of products, such as high ZnS type, apparent treatment type, water dispersion type, light type, color type and so on, and plays an important role in industrial and agricultural production and daily life. It is mainly used for coating, paint, thermoplastic, thermosetting plastics, paper and so on. Through the surface treatment or coating, it can replace part of rutile.
At present, lithopone is produced mainly in China. The production method of lithopone is traditional method in domestic, the main raw material is the low-grade zinc oxide, sulfate and barium sulfide (prepared by high temperature reducing using barite and coal), the zinc content of the raw materials is 45%˜70%. The traditional process for production of lithopone zinc contains more than 45% zinc calcine as raw materials, leaching by Sulfuric acid, to obtain the coarse liquid of zinc sulfate, and then removing iron by oxidation method using the potassium permanganate, and then replace the heavy metals using zinc powder, filtering to obtain refined liquid of zinc sulfate, and then carrying out metathetical reaction with barium sulfide, pressure filtering, calcining, rinsing, drying, crushing, and obtain different types of lithopone product series with more than 30% of zinc sulfide. The whole process is in acid (pH<7) environment, so the whole process consumes a large amount of sulfuric acid, which has a strong corrosive. It has high requirements for production equipment, and the discharge of these present methods is acid residue, so they bring new pollution, furthermore, it requires high-grade raw materials and has high cost of production, the obtained products quality is poor.
With the development of science and technology, it has developed many kinds of pigment, inorganic pigment is facing serious challenges. Nanometer material is a kind of new material which is developed at the end of the twentieth century and it is with mesoscopic dimension scale, and it direct at low dimensional and composite. In recent years, nanometer lithopone has been put into industrial production, and it was prepared by zinc sulfate solution with the traditional method for the same, and it was synthesized in organic phase (such as benzene). These methods have high production cost, and are difficult to recycle waste water, organic solvent in the process of production and final leaching residue are easy to cause pollution to the environment, its development is severely restricted.
In addition, along with the demand of zinc compounds and lithopone is more and more big. With the continuous mining of zinc resources in our country, mineral resources become depleted, small and complex, comprehensive utilization and environmental protection requirements continue to improve, people have begun to study the utilization technology of recovery of low content zinc material. Said low content zinc material is mainly low-grade zinc oxide ore. However, in addition to low-grade zinc oxide ore contains zinc, acid leaching residue also contains zinc.
At present, in domestic zinc hydrometallurgy mainly adopts the production process of roasting, leaching and electrowinning. In the acid leaching residue which is discharged by said production process, the mass percent content of zinc is in general 8˜15%, some as high as 20%, the mass percent content of sulfur is 6-12% and the mass percent content of sulfate radical is 15-30%, which mainly in the form of CaSO4 (sulfate leaching rate can reach to 70% in the ammonia environment and multi-segment extraction). In the acid leaching residue, zinc exists mainly in the form of ZnFe2O4. In order to recycle this zinc, the current treatment methods are pyrometallurgy and hydrometallurgy. Said pyrometallurgy contains kiln volatilization method (namely Waelz process) and fuming furnace volatilization method. Said hydrometallurgy contains hot acid leaching and high temperature & pressure leaching. Pyrometallurgy has the disadvantages of long process, a large quantity equipment repair, high investment, and poor working environment, need to consume a large amount of coal or coal for metallurgy, low efficiency, environmental pollution. Therefore, it usually adopts hot acid leaching or high pressure leaching, these methods still have disadvantages as follow: 1. consuming a large amount of acid, low leaching rate, because it contains large amount of calcium sulfate, calcium sulfate is ultrafine particle which effect on membrane isolation for the zinc oxide particles. It result that it is difficult to leach zinc in acid environment. Twice weak acid leaching due to low recovery rate, it has little practical significance. 2. If leaching by strong acid, although ZnFe2O4 has been destroyed, the leaching rate increased, the leaching rate of iron is also high (up to 60%), it also has pressure of removing iron, and consumes more reagents, 3. the high temperature and high pressure bring serious equipment corrosion, complex equipment and the investment is large; 4. the operation cost is high, and the economic benefit is poor; 5. finally, the discharge of these present methods is acid residue, so they bring new pollution, it has no choice but to bury, not only pollute the environment, but also a waste of resources.
Therefore, the most ideal processing method for the acid leaching residue is the selective leaching of zinc, it make the zinc entering into the final leached solution, and recycle zinc to prepare valuable zinc products such as lithopone etc.
In summary, for the process of electrolytic zinc acid leaching residue, how to overcome the disadvantage of traditional pyrometallurgy and hydrometallurgy, leaching out zinc in low zinc content material effectively, and obtaining the nanometer lithopone, become the technical problem need to solves urgently.